The present invention relates generally to the control of tools used downhole in a subterranean wellbore and more particularly provides, in various illustratively depicted embodiments thereof, specially designed annular plug seat assemblies having circumferentially overlapping seat segment joints.
A common practice for controlling various types of tools downhole, such as for example, sliding sleeve valves, is to use pressurized fluid to flow a ball (or other type of plug structure) down the wellbore to land on a generally annular seat structure operatively associated with the particular tool. When the ball lands on the seat, it blocks fluid from flowing in a downhole direction through the seat, thereby creating a pressure drop across the seat that may be utilized to create a control event such as shifting a sliding sleeve valve.
Many seat configurations have been previously proposed, the most simplistic of which being a solid ring with an inner diameter smaller than the ball's diameter. Seats capable of expanding to let the ball pass therethrough have also been previously proposed. These seats incorporated a collet-like structure, or radial dogs contained in an axially movable sleeve, and when engaged by a ball were slidable to a further downhole position at which diametrical expansion of the collet or dogs was permitted to allow the ball to pass through the seat. Such previously proposed seat designs often proved to be problematic since they have inherent gaps that could be infiltrated by sand, mud, cement or grit often present in the well. These gaps between the circumferential seat segments could be present when balls were not passing through the seats, thus enabling the infiltrating contaminants to cause system seize-up.
Another previously proposed diametrically expandable annular seat design, illustrated and described in copending U.S. patent application Ser. No. 13/887,779 filed May 6, 2013 and assigned to the assignee of the present invention, does not incorporate a collet-like structure, or have radially sliding dogs contained within an axially moveable sleeve. Instead, a circumferentially segmented annular seat rests against a conical shoulder rigidly affixed to the tool. The seat is diametrically compressed from the opposing side with a conically engaging sleeve that is biased against the seat via a spring or by fluid pressure. In this seat design there are no gaps for contamination to penetrate during periods when balls are not passing through the seat. This is especially important when hydraulically fracturing a well since cement and proppant would certainly penetrate such gaps. Conveniently, when hydraulically fracturing a well with a sliding sleeve ball drop system, the practice is typically to remove the slurry while pumping down a ball to create a pad of clean water around the ball. Consequently, momentary gaps while the ball passes do not see proppant, mud, or cement. This practice is primarily used to reduce the chance of an undesirable screen-out condition.
While this last-mentioned annular plug seat design has been found to be generally satisfactory for its intended purposes, and superior in performance to seats with collet or dog configurations, it has also been found that a single plane axially extending interface provided between each circumferentially adjacent seat segment pair may, in some instances such as when the balls are pumped downhole at higher speeds, cause the seat to malfunction. It is desirable to pump balls at a fast rate since pumping down too slowly can cause the proppant to fall out of suspension with the associated fluid. When balls are pumped down at too great a speed, the simple single plane interfaces between each circumferentially adjacent seat segment pair may allow individual seat segments to be washed into the bore in front of the ball. Such seat segment washout (in which a segment is axially separated from the balance of the seat) typically causes complete collapse of the seat and/or seizure of the non-washed out segments in a manner preventing balls from passing through the remainder of the seat.
As can be seen from the foregoing, a need exists for an improved annular downhole tool plug seat structure that eliminates or at least substantially alleviates the above-mentioned problems, limitations and disadvantages of previously proposed seat designs as generally described above. It is to this need that the present invention is primarily directed.